The development of high energy battery systems requires, among other things, the compatibility of an electrolyte solution possessing desirable electrochemical properties with highly reactive anode materials, such as sodium and lithium. The use of aqueous electrolytes is precluded in these systems since the anode materials are sufficiently active to react with water chemically. It has, therefore, been necessary, in order to realize the high energy density obtainable through use of these highly reactive anodes, to turn to the investigation of non-aqueous electrolyte systems.
The term "non-aqueous electrolyte" as used herein refers to an electrolyte solution which is composed of a solute such as, for example, a metal salt or a complex salt of Group I-A, Group II-A or Group III-A elements of the Periodic Table, dissolved in an appropriate nonaqueous solvent. The term "Periodic Table" as used herein refers to the Periodic Table of Elements as set forth on the inside front cover of the Handbook of Chemistry and Physics, 60th Edition, CRC Press Inc., Boca Raton, Florida, 1979-1980.
A multitude of solutes is known and many have been suggested for use but the selection of a suitable solvent has been particularly troublesome. The ideal battery electrolyte would comprise a solvent-solute pair which has a long liquid range, high ionic conductivity and stability. A long liquid range, i.e., high boiling point and low freezing point, is essential if the battery is to operate at other than normal ambient temperatures. High ionic conductivity is necessary if the battery is to have high rate capability. Stability is necessary with the electrode materials, the materials of cell construction and the products of the cell reaction to provide long shelf life when used in a primary or secondary battery system.
It has been disclosed in the literature that certain materials are capable of acting both as an electrolyte carrier, i.e., as solvent for the electrolyte salt, and as the active cathode for a non-aqueous electrochemical cell. U.S. Pat. Nos. 3,475,226, 3,567,515 and 3,578,500 each disclose that liquid sulfur dioxide or solutions of sulfur dioxide and a cosolvent will perform this dual function in non-aqueous electrochemical cells. While these solutions perform their dual function, they are not without several disadvantages in use. Sulfur dioxide is always present and, being a gas at ordinary temperatures, it must be contained in the cell as a liquid under pressure or dissolved in a liquid solvent. Handling and packaging problems are created if the sulfur dioxide is used alone, and an additional component and assembly step are necessary if sulfur dioxide is to be dissolved in a liquid solvent. As stated above, a long liquid range encompassing normal ambient temperature is a desirable characteristic in an electrolyte solvent. Obviously, sulfur dioxide is deficient in this respect at atmospheric pressure.
U.S. application Ser. No. 439,521 by G. E. Blomgren et al, filed Feb. 4, 1974, discloses a non-aqueous electrochemical cell comprising an anode, a cathode collector and a cathode-electrolyte, said cathode-electrolyte comprising a solution of an ionically conductive solute dissolved in an active cathode depolarizer wherein said active cathode depolarizer consists of a liquid oxyhalide of an element of Group V or Group VI of the Periodic Table.
U.S. application Ser. No. 474,267 by G. E. Blomgren et al, filed May 29, 1974, discloses a non-aqueous electrochemical cell employing a cathode-electrolyte consisting of an ionizing solute dissolved in a liquid halide solvent and a cosolvent, said liquid halide being selected from the group consisting of sulfur monochloride (S.sub.2 Cl.sub.2), sulfur monobromide (S.sub.2 Br.sub.2), selenium tetrafluoride (SeF.sub.4), selenium monobromide (Se.sub.2 Br.sub.2), thiophosphoryl chloride (PSCl.sub.3), thiophosphoryl bromide (PSBr.sub.3), vanadium pentafluoride (VF.sub.5), lead tetrachloride (PbCl.sub.4), titanium tetrachloride (TiCl.sub.4), disulfur decafluoride (S.sub.2 F.sub.10), tin bromide trichloride (SnBrCl.sub.3), tin dibromide dichloride (SnBr.sub.2 Cl.sub.2) and tin tribromide chloride (SnBr.sub.3 Cl).
One of the objects of the present invention is to provide a new group of liquid cathodic materials that have a long liquid range for use in nonaqueous cells.
Another object of the present invention is to provide a new group of liquid cathodic materials that are relatively stable with anodes such as lithium and have a relatively low reactivity with atmospheric water vapor thus making them suitable for use in non-aqueous cell systems.
The foregoing and additional objects will become more fully apparent from the following description.